Kibble–Zurek universality in a strongly interacting Fermi superfluid

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The Kibble–Zurek mechanism describes the spontaneous formation of topological defects in a system crossing a continuous phase transition1,2. Its central premise is the notion of universality, which states that the characteristic scaling exponent describing the dependence of the defect density on the quench rate is determined by the underlying symmetries of the system. Whether this universality can be extended to strongly interacting systems, such as a unitary Fermi gas, is an open question that has recently drawn attention in the context of holographic theories3,4. Here, we report the observation of the Kibble–Zurek universality in a strongly interacting Fermi superfluid. As the microscopic nature of superfluidity is tuned from Bose–Einstein condensation of tightly bound molecules to Bardeen–Cooper–Schrieffer superfluidity of long-range fermion pairs, the thermal quench formation of vortices reveals a constant scaling exponent arising from the U(1) gauge symmetry of the system. In rapid quenches, destructive vortex collisions lead to the saturation of vortex densities, the values of which can be universally scaled by the interaction-dependent area of the vortex cores. This work paves the way for precision studies of non-equilibrium dynamics in a highly tunable, strongly correlated many-fermion system5,6.

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Fig. 1: Spontaneous defect formation during the normal-to-superfluid phase transition in a strongly interacting Fermi gas.
Fig. 2: Vortex number versus quench time.
Fig. 3: Kibble–Zurek exponents in the BEC–BCS crossover.
Fig. 4: Characterization of spontaneous vortex formation across the BEC–BCS crossover.

Data availability

The data supporting this manuscript are available from the corresponding authors on reasonable request.


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We thank A. del Campo for discussions. This work was supported by the Institute for Basic Science in Korea (grant no. IBS-R009-D1) and the National Research Foundation of Korea (grant no. NRF-2018R1A2B3003373). J.W.P. acknowledges support from the POSCO Science Fellowship of the POSCO TJ Park Foundation.

Author information

J.W.P. and Y.S. conceived the idea. B.K. and J.W.P. performed the experiment and data analysis. J.W.P. and B.K. wrote the manuscript, and all authors discussed the results and commented on the manuscript. J.W.P. and Y.S. supervised the project.

Correspondence to Jee Woo Park or Y. Shin.

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Supplementary Figs. 1–5, refs. 1–5 and text.

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Ko, B., Park, J.W. & Shin, Y. Kibble–Zurek universality in a strongly interacting Fermi superfluid. Nat. Phys. (2019) doi:10.1038/s41567-019-0650-1

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